DNA aptamers are single-stranded DNA molecules with defined tertiary structures for target binding. Aptamers recognize molecules other than nucleic acids with high affinity and specificity. Aptamers have been identified which can recognize a broad range of targets, including small molecules and large proteins. Aptamers can be produced synthetically and are very stable. They have the potential of being used as effective molecular tools for many applications including medical diagnosis, biosensing and drug screening.
One method for the selection of an aptamer that specifically binds to a target molecule is the SELEX process described in U.S. Pat. Nos. 5,475,096 and 5,270,163 (incorporated herein by reference). Briefly, the SELEX involves immobilizing a selected target on a column. A solution containing a library or assortment of random DNA or RNA molecules is contacted with the target under conditions that are favorable for binding. After a certain amount of time the column is flushed and those nucleotide molecules which bind to the target remain on the column. The unbound nucleic acid molecules will be eluted from the column. The nucleic acid-target complexes are then dissociated and the nucleic acid molecules that were selected by binding to the target are amplified. The cycle may be repeated to achieve a higher affinity nucleic acid.
In copending PCT application PCT/CA02/02007, which is incorporated herein by reference, the present inventors disclose a method for transforming a non-signaling aptamer into a fluorescent one based on the ability of aptamers to switch structure. In the absence of its cognate non-nucleic acid target, an aptamer forms a duplex structure with a complimentary nucleic acid molecule. In the presence of the target the nucleic acid duplex structure is dissociated and the aptamer assumes its tertiary structure.
Techniques that allow for the specific detection of molecules are highly sought after for the development of research and clinical tests. Thus, there remains a real need for more rapid and efficient methods for the identification and isolation of aptamers that are specific for particular targets.